Please help our simple bgp

Joel Maslak jmaslak at
Tue Jan 31 03:06:14 UTC 2012

On Mon, Jan 30, 2012 at 7:27 PM, Ann Kwok <annkwok80 at> wrote:

> We discover the routes is going to ISP A only even the bandwidth 100M is
> full

There are several ways to handle this is, if you have at least two
/24s of space.

Let's say you just have two /24s, both part of the same /23.

Option 1:

Announce m.m.m.m/24 with no path prefixing on ISP A.
Announce m.m.m.m/24 prefixed with your own ASN one or two times on ISP B.
Announce n.n.n.n/24 with no path prefixing on ISP B
Announce n.n.n.n/24 prefixed with your own ASN one or two times on ISP A.

Most of the internet would probably prefer A for m.m.m.m/24, and
prefer B for n.n.n.n/24.  But if either A or B went down, there would
still be a reachable route.

Option 2:

Announce m.m.m.m/23 on both ISP A and B
Announce m.m.m.m/24 on ISP A
Announce n.n.n.n/24 on ISP B

The n.n.n.n/24 which is part of m.m.m.m/23 would use ISP B for inbound
traffic, while ISP A would be used for m.m.m.m/24.  If either A or B,
the less specific /23 would provide a backup path.

> Can we set the weight to change to ISP B to use ISP B as preference routes?

Not really.  You may be able to set a community that controls how ISP
B advertises the routes or preferences your traffic.  Weights
generally aren't used for path selection.

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